PPP calls Collins one of the most popular senators in the country. The Republican has at least 60 percent favorability among Republicans, Democrats and independents.

Collins faces her only potential problem in a primary challenge from a more conservative Republican. In PPP’s test of Collins against a theoretically “more conservative” Republican, Collins had a slim 49-46 edge.

But if Republicans dumped Collins in a primary or if she decided against running for re-election, PPP found Republicans’ chances of retaining Collins’ seat aren’t good. PPP tested former Secretary of State Charlie Summers, who challenged Sen. Angus King last year in the race to replace Olympia Snowe, against Michaud and Pingree and found he trails both incumbent U.S. House members.

Neither Michaud nor Pingree has publicly indicated an interest in running for Collins’ seat. Their names circulate more often as potential candidates for governor in 2014.

PPP surveyed 1,268 Maine voters for the survey, which had a 2.8 percent margin of error. PPP also surveyed 430 usual Republican primary voters for a Republicans-only poll, which had a 4.7 percent margin of error.